Ion selectivity in capacitive deionization with functionalized electrode: Theory and experimental validation

Abstract

Capacitive deionization (CDI) is a promising technique for salt removal and may have potential for highly selective removal of ion species. In this work, we take advantage of functional groups usually used with ionic exchange resins and apply these to CDI. To this end, we functionalize activated carbon with a quaternary amines surfactant and use this surface to selectively and passively (no applied field) trap nitrate ions. We then set the cell voltage to a constant value to regenerate these electrodes, resulting in an inverted capacitive deionization (i-CDI) operation. Unlike resins, we avoid use of concentrated chemicals for regeneration. We measure the selectivity of nitrate versus chloride ions as a function of regeneration voltage and initial chloride concentration. We experimentally demonstrate up to about 6.5-fold (observable) selectivity in a cycle with a regeneration voltage of 0.4 V. We also demonstrate a novel multi-pass, air-flush i-CDI operation to selectively enrich nitrate with high water recovery. We further present a dynamic, multi-species electrosorption and equilibrium solution-to-surface chemical reaction model and validate the model with detailed measurements. Our i-CDI system exhibits higher nitrate selectivity at lower voltages; making it possible to reduce NaNO 3 concentrations from ∼170 ppm to below the limit of maximum allowed values for nitrate in drinking water of about 50 ppm NaNO 3 .

Capacitive processes for carbon capture and energy recovery from CO2 emissions : Shaping a new technology going from water to gas applications

“…The first model is the so-called amphoteric Donnan (amph-D) model developed by Gao et al 165 for conventional CDI, while the second model is the multiequilibria amphoteric Donnan (m-amph-D) model. The m-amph-D model is based on the model proposed by Hemmatifar and Oyarzun et al 119,174 for conventional CDI. The amph-D model is an extended version of the Donnan model 84 , which considers the presence of immobile chemical charge on the carbon surface, the amount of which is assumed to be constant and not dependent on local (electrical) charge and pH.…”

“…In this section, we theoretically investigate the adsorption mechanisms by comparing the amphoteric Donnan (amph-D) model 86,165 with experimental data. To the best of our knowledge, the amph-D model has been extensively used for NaCl solutions 79,169,174,177,178 , but never for CO2-sparged solutions. We performed CO2-CDI experiments with two different carbon materials, i.e., activated carbon (AC) and carbon cloth (CC) electrodes.…”

“…Refs. 174,177 study the effect of chemisorption of specific chemical species (mostly NO3 -) on ion adsorption and include a new parameter in the amph-D model to describe this effect (intrinsic selectivity coefficient 174 or affinity term 177 ). Fig.…”

“…Other effects not considered in this study could also influence the absorption efficiency, e.g., (i) the ion size-based selectivity 174 and (ii) the non-ideal dissociation degree in the micropores of the EDL. Regarding size-based selectivity, monovalent ions with lower hydrated radius (i.e., K + and Na +179 ) and a lower ion hydration ratio 180 have shown to be selectively adsorbed in the electrode EDLs from monovalent salt mixtures (e.g., with K + and Na + ).…”

“…While fuel cell is a mature technology, which has been actively developed for many years, CDI is still a young technology with untapped potential. CDI technology is developing fast and is finding continually more applications along with its development (e.g., in water desalination, selective ions removal 174,177 , sterilization 117 , ammonia recovery 247 ). CO2-CDI is now a new member of the CDI technology family, which contributes to the scientific understanding of CDI cells.…”